Fix static analysis false-positives

* It couldn't detect that the `fastCoverParams` can't be non-null, since it was just an assertion.
* It thought we were accesing `wksp->dtable` beyond the bounds because we were using it to set the `workSpace` value. Instead, compute the workspace size used in a different way.

lib/common/fse.h

@@ -353,7 +353,7 @@ size_t FSE_buildDTable_raw (FSE_DTable* dt, unsigned nbBits);
 size_t FSE_buildDTable_rle (FSE_DTable* dt, unsigned char symbolValue);
 /**< build a fake FSE_DTable, designed to always generate the same symbolValue */
 
-#define FSE_DECOMPRESS_WKSP_SIZE_U32(maxTableLog, maxSymbolValue) (FSE_DTABLE_SIZE_U32(maxTableLog) + FSE_BUILD_DTABLE_WKSP_SIZE_U32(maxTableLog, maxSymbolValue) + (FSE_MAX_SYMBOL_VALUE + 1) / 2 + 1)
+#define FSE_DECOMPRESS_WKSP_SIZE_U32(maxTableLog, maxSymbolValue) (FSE_DTABLE_SIZE_U32(maxTableLog) + 1 + FSE_BUILD_DTABLE_WKSP_SIZE_U32(maxTableLog, maxSymbolValue) + (FSE_MAX_SYMBOL_VALUE + 1) / 2 + 1)
 #define FSE_DECOMPRESS_WKSP_SIZE(maxTableLog, maxSymbolValue) (FSE_DECOMPRESS_WKSP_SIZE_U32(maxTableLog, maxSymbolValue) * sizeof(unsigned))
 size_t FSE_decompress_wksp(void* dst, size_t dstCapacity, const void* cSrc, size_t cSrcSize, unsigned maxLog, void* workSpace, size_t wkspSize);
 /**< same as FSE_decompress(), using an externally allocated `workSpace` produced with `FSE_DECOMPRESS_WKSP_SIZE_U32(maxLog, maxSymbolValue)` */

lib/common/fse_decompress.c

@@ -342,7 +342,8 @@ FORCE_INLINE_TEMPLATE size_t FSE_decompress_wksp_body(
     }
 
     if (FSE_DECOMPRESS_WKSP_SIZE(tableLog, maxSymbolValue) > wkspSize) return ERROR(tableLog_tooLarge);
-    workSpace = wksp->dtable + FSE_DTABLE_SIZE_U32(tableLog);
+    assert(sizeof(*wksp) + FSE_DTABLE_SIZE(tableLog) <= wkspSize);
+    workSpace = (BYTE*)workSpace + sizeof(*wksp) + FSE_DTABLE_SIZE(tableLog);
     wkspSize -= sizeof(*wksp) + FSE_DTABLE_SIZE(tableLog);
 
     CHECK_F( FSE_buildDTable_internal(wksp->dtable, wksp->ncount, maxSymbolValue, tableLog, workSpace, wkspSize) );

programs/dibio.c

@@ -31,6 +31,7 @@
 #include "timefn.h"         /* UTIL_time_t, UTIL_clockSpanMicro, UTIL_getTime */
 #include "../lib/common/debug.h" /* assert */
 #include "../lib/common/mem.h"  /* read */
+#include "../lib/zstd_errors.h"
 #include "dibio.h"
 
 
@@ -380,7 +381,7 @@ int DiB_trainFromFiles(const char* dictFileName, size_t maxDictSize,
         srcBuffer, &loadedSize, sampleSizes, fs.nbSamples, fileNamesTable,
         nbFiles, chunkSize, displayLevel);
 
-    {   size_t dictSize;
+    {   size_t dictSize = ZSTD_error_GENERIC;
         if (params) {
             DiB_fillNoise((char*)srcBuffer + loadedSize, NOISELENGTH);   /* guard band, for end of buffer condition */
             dictSize = ZDICT_trainFromBuffer_legacy(dictBuffer, maxDictSize,
@@ -400,8 +401,7 @@ int DiB_trainFromFiles(const char* dictFileName, size_t maxDictSize,
               dictSize = ZDICT_trainFromBuffer_cover(dictBuffer, maxDictSize, srcBuffer,
                                                      sampleSizes, nbSamplesLoaded, *coverParams);
             }
-        } else {
+        } else if (fastCoverParams != NULL) {
-            assert(fastCoverParams != NULL);
             if (optimize) {
               dictSize = ZDICT_optimizeTrainFromBuffer_fastCover(dictBuffer, maxDictSize,
                                                               srcBuffer, sampleSizes, nbSamplesLoaded,
@@ -416,6 +416,8 @@ int DiB_trainFromFiles(const char* dictFileName, size_t maxDictSize,
               dictSize = ZDICT_trainFromBuffer_fastCover(dictBuffer, maxDictSize, srcBuffer,
                                                         sampleSizes, nbSamplesLoaded, *fastCoverParams);
             }
+        } else {
+            assert(0 /* Impossible */);
         }
         if (ZDICT_isError(dictSize)) {
             DISPLAYLEVEL(1, "dictionary training failed : %s \n", ZDICT_getErrorName(dictSize));   /* should not happen */